1. Field of the Invention
The present invention relates to fastening devices applied to rotary machines such as gas turbines and steam turbines and, in particular, to a fastening device applied to a portion of a rotary machine that is acted on by a centrifugal force in the direction perpendicular (orthogonal) to a fastening direction.
This application is based on Japanese Unexamined Patent Application, Publication No. 2008-128292, the content of which is incorporated herein by reference.
2. Description of Related Art
A known example of a fastening device applied to a portion of a rotary machine that is acted on by a centrifugal force in the direction perpendicular to a fastening direction is disclosed in Japanese Unexamined Patent Application, Publication No. 2001-65586.
However, the fastening device disclosed in Japanese Unexamined Patent Application, Publication No. 2001-65586 has a problem in that the contact pressure generated between the back surface of a nut located radially outside and the surface of an object to be fastened, opposing this back surface, is increased and the contact pressure generated between the back surface of the nut located radially inside and the surface of the object to be fastened, opposing this back surface, is decreased by a centrifugal force that acts on the nut. This increases the tightening load at the engaged portion between the thread of the nut and the thread of a fastening bolt at the beginning of the engagement and decreases it at the end of the engagement, making it uneven along the length of the thread, so that a stress is concentrated in the vicinity of the beginning of the engagement.
A load distribution will be described with reference to FIGS. 9 and 10 in the case where a bending moment due to a tensile stress and a centrifugal force acts on a fastening bolt.
As shown at the upper part in FIG. 9, when a simple tensile load acts on a fastening bolt 201, a load distribution is generated along the axis of the fastening bolt 201. In this case, the maximum load acts at the beginning of the engagement (the starting point of the engagement) of the fastening bolt 201, at which the maximum stress is applied. When a centrifugal force further acts in this state, a bending moment due to the centrifugal force of a nut 202 exerts a raising action on the fastening bolt 201. Therefore, as shown at the left of FIG. 9 and FIG. 10, an uneven circumferential load distribution is generated at the end of the engagement (engaging end) of the fastening bolt 201. FIG. 10 shows a load distribution of the fastening bolt 201 in a cross-sectional view.
On the other hand, the nut 202 is acted on by a tightening force between it and an object to be fastened 203, so that the centrifugal force of the nut 202 is offset against a frictional force due to this tightening force. Its contact pressure exhibits the distribution as shown in the right of FIG. 9. If the centrifugal force of the nut 202 is larger than the frictional force, the excess centrifugal force acts to push the thread of the fastening bolt 201 upward and is transmitted to the fastening bolt 201. This pushing force becomes a maximum at the 6° clockwise position of the fastening bolt 201 in a cross-sectional view. This excess centrifugal force is finally transmitted to the main body of the fastening bolt 201 as a shearing force that acts on the cross section of the fastening bolt 201 at the beginning of the engagement. These pushing force and shearing force that act on the screw surface are factors causing an increase in the degree of concentration of the stress at the maximum contact pressure load position shown at the right of FIG. 9.